Devices with thinned wafer

ABSTRACT

Methods, apparatuses and devices are described where a main wafer is irreversibly bonded to a carrier wafer and thinned to reduce a thickness of the main wafer, for example down to a thickness of 300 μm or below.

FIELD

The present application relates to methods and apparatuses formanufacturing devices, for example micro-electro-mechanical systems(MEMS), and to corresponding devices.

BACKGROUND

Micro-electro-mechanical systems are generally manufactured by formingmechanical structures in a wafer, typically a semiconductor wafer like asilicon wafer. Electrical structures may be formed on the same wafer. Inorder to decrease the volume of such micro-electro-mechanical systems,it would be desirable to reduce the thickness of the structured MEMSwafer. However, thicknesses below about 400 μm are difficult to obtaindue to stability problems.

In other fields of technology than manufacturing MEMS, for processingthinned wafers the wafers may be mounted to a carrier using for examplea glue or adhesive and released from the carrier after processing.However, at least for some kinds of micro-electro-mechanical systems,this may not be feasible as the wafer may be structurally weakened dueto the formation of the mechanical structures. This in turn may lead todamages to the wafer when the wafer is released from the carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative examples of embodiments will be described with reference tothe attached drawings in the detailed description that will follow.

FIG. 1 is a block diagram of an apparatus according to an embodiment.

FIG. 2 is a flowchart illustrating a method according to an embodiment.

FIGS. 3A to 3K are schematic cross-sectional views of a device accordingto an embodiment in various stages of a manufacturing process thereof.

FIGS. 4A and 4B are schematic cross-sectional views illustratingalternative manufacturing techniques.

FIG. 5 is a schematic cross-sectional view of a device according to anembodiment.

DETAILED DESCRIPTION

In the following, various illustrative embodiments will be described indetail. It is to be understood that these embodiments serve as examplesonly and are not to be construed as limiting the scope of the presentapplication. For example, while embodiments may be described ascomprising a plurality of features or elements, in other embodimentssome of these features or elements may be omitted and/or replaced byalternative features or elements. Additionally or alternatively, in someembodiments additional features or elements apart from the onesexplicitly described may be provided. Moreover, while some specificexamples for micro-electro-mechanical systems will be given to provide aclearer understanding, it is to be noted that techniques disclosedherein may also be applicable to other micro-electro-mechanical systemsor of the devices, for example electronic devices.

In the following, the terms “wafer” and “substrate” may be usedinterchangeable to refer to pieces of plate-like material. The materialis essentially arbitrary and may for example comprise glass or asemiconductor like silicon. Wafers or substrates may be processed orunprocessed. Processing of a wafer may comprise modifying the form ofthe substrate, for example by etching, forming devices, for examplesemiconductor devices, on or in the substrate, modifying a doping of thesubstrate or providing layers like oxide layers or metal layers on or inthe substrate. According to some embodiments, a micro-electro-mechanicalsystem is provided comprising a processed wafer implementing thefunctionality of the micro-electro-mechanical system, a thickness ofthis wafer being below 300 μm, for example below 100 μm, for examplebelow 50 μm. The wafer in which at least the greatest part of thestructures relevant to the functioning of the micro-electro-mechanicalsystem are formed will also be referred to as “main wafer” in thefollowing. The main wafer may be irreversibly bonded to a first carrierwafer on one side thereof. In some embodiments, additionally, thesemiconductor wafer may be irreversibly bonded to a second carrier waferon another side thereof. The first and/or second carrier wafers may forexample be made of glass. In other embodiments, other materials may beused for the first and/or second carrier wafers, for example silicon(Si), germanium (Ge), a silicon germanium crystal (SiGe), siliconcarbide (SiC), gallium arsenide (GaAs), gallium nitride (GaN) or anothertype of III-V semiconductor. In embodiments, the material of the firstand/or second carrier wafers may be selected to have a thermal expansioncoefficient similar to a thermal expansion coefficient of the mainwafer.

“Irreversibly bonded” in the context of the present application mayindicate that it is essentially impossible to release the bonding in anon-destructive manner. This is in contrast for example to a bonding ofa wafer to a carrier using glue or adhesive, which may be dissolved e.g.by irradiation with ultraviolet (UV) light, applying an increasedtemperature or applying a solvent, thus reversing the bonding basicallywithout destroying or damaging anything on the wafer.

In some embodiments, such a micro-electro-mechanical system may beformed by irreversibly bonding a preprocessed wafer as main wafer to acarrier wafer like a glass wafer, followed by a thinning and a formationof structures, in particular mechanical structures or cavities, in thesemiconductor wafer. In other embodiments, other wafers thansemiconductor wafers may be used.

The above embodiments give only a short overview of some features ofsome embodiments and are not to be construed as limiting.

In FIG. 1, a schematic block diagram of an apparatus according to anembodiment for manufacturing a micro-electro-mechanical system is shown.The term “apparatus” in this respect is not to be construed asindicating any spatial configuration of the apparatus. In particular,the apparatus will be described as comprising a plurality of differentdevices for carrying out different types of processing. These devicesneed not be located close to each other, but may also be located remotefrom each other, for example in different rooms, buildings or even indifferent locations, as long as wafers may be transported from onedevice to the next in an appropriate manner, for example withoutinducing unwanted contaminations. Furthermore, while individual devicesare shown as blocks, they may consist of several sub-devices which alsoneed not be located close to each other. Furthermore, some of thedevices may share components or may be implemented in a single device.

As indicated by arrows 15 and 16 in FIG. 1, prior to being processed bydevices 10 to 14 shown in FIG. 1, and after being processed by devices10 to 14, substrates and wafers my be processed by further (not shown)devices, which may be any kind of devices conventionally used forprocessing of wafers and substrates. In some embodiments, also betweendevices 10 to 14 other devices (not shown) to provide some conventionalprocessing may be provided.

In the embodiment of FIG. 1, a wafer (main wafer) which may haveundergone previous processing is provided to a first material removaldevice 10. In first material removal device 10, some material may beselectively removed from the main wafer, for example at locations wherean opening or space reaching through the complete wafer is to be formedlater. For example, material of an epitaxial layer (epilayer) and/or ofan oxide layer may be removed.

In some embodiments, the main wafer may be a semiconductor wafer.

After having been processed by the first material removal device 10, inthe embodiment of FIG. 1 the main wafer is forwarded to a filling device11. In filling device 11, voids or gaps left by the removal of materialin first material removal device 10 may be filled with a fillingmaterial. In some embodiments, as filling material a material removableby a dry process, for example a carbon-based material, removable by e.g.an oxygen (O₂) plasma, may be used. In embodiments where no openingthrough the complete main wafer has to be formed, devices 10 and 11 maybe omitted.

After the filling performed by filling device 11, the wafer is providedto a carrier wafer bonding device 12. In carrier wafer bonding device12, a carrier wafer, for example a glass wafer, which may be structured,is bonded to the main wafer, for example at a side where material wasremoved in first material removal device 10 and filled in filling device11 (e.g. a front side). The bonding may be an irreversible bonding, forexample, an anodic bonding or a hermetic bonding via ceramic adhesivesor glass solder.

Next, the main wafer together with the bonded carrier wafer is fed to athinning device 13, where the main wafer is thinned, for example to athickness below 300 μm, below 100 μm, below 50 μm or less. The thinningmay for example be performed by mechanical means like grinding and/orpolishing, and/or by etching.

Next, material is selectively removed from the main wafer in a secondmaterial removal device 14, for example to contribute forming structuresfor a micro-electro-mechanical system, for example cavities, membranes,cantilevers, tongues or the like. In some embodiments, in secondmaterial removal device 14 also the filling material filled in fillingdevice 11 may be removed.

As already mentioned, after second material removal device 14, furtherprocessing may occur in further devices. For example, a further carrierwafer may be bonded to the main wafer.

In FIG. 2, a flowchart illustrating a method according to an embodimentis illustrated. The method of FIG. 2 may be implemented using theapparatus illustrated and described with respect to FIG. 1, but may alsobe implemented independently therefrom. Modifications discussed withrespect to the apparatus of FIG. 1 may also apply to the method of FIG.2 discussed below and vice versa.

At 21, material is removed from a front side of the main wafer. A frontside in embodiments may be a side of the main wafer where structures, inparticular electrical structures, are formed, for example resistors orelectrical contacts. As indicated by an arrow 20, such structures may beformed in prior processing before applying the method of FIG. 2, forexample by employing conventional semiconductor processing techniques.

In some embodiments, material, for example part(s) of an epilayer and/oran oxide layer is removed at places where openings are to be providedthrough the main wafer in a micro-electro-mechanical system (MEMS) to beproduced.

At 22, one or more gaps generated by the material removal at 21 may befilled with a temporary filling material which is intended to be removedagain later. In some embodiments, the temporary material is selected tobe removable by a dry process like a dry etching process or an oxygen(O₂) plasma, also referred to as oxygen flash. In some embodiments, acarbon-based material may be used as filling material.

At 23, the main wafer is bonded to a carrier wafer, for example a glasswafer. The bonding may take place at the front side of the main waferand may be an irreversible bonding, for example anodic bonding.

At 24, the main wafer is thinned, for example from a back side of mainwafer opposite to the front side. As the main wafer is bonded to thecarrier wafer at 23, in some embodiments this may give sufficientstability and support to the main wafer for the thinning. In someembodiments, an original thickness of the main wafer may be betweenabout 400 μm and about 725 μm, although other thicknesses may alsoapply. Through the thinning at 24, the main wafer is for example thinnedto a thickness at or below 300 μm, at or below 100 μm, at or below 50 μmor even less.

At 25, material is removed from the backside of the main wafer, forexample to form structures like cavities in the main wafer necessary fora functioning and/or forming of a mechanical part of themicro-electro-mechanical system to be formed. In some embodiments, at25, additionally, the filling material filled at 22, is removed.

After 25, as indicated by an arrow 26, further processing may takeplace, for example a bonding of a further carrier wafer to the mainwafer, for example at a backside thereof. Also the further carrier wafermay comprise a structured glass wafer, but is not limited thereto.

To further illustrate the apparatus of FIG. 1 and the method of FIG. 2,with respect to FIGS. 3A to 3K, a micro-electro-mechanical system invarious stages of processing is shown. At least some of these stages mayillustrate processing performed by the devices of FIG. 1 and/orprocessing using the method of FIG. 2. It is to be emphasized that thesespecific examples of FIGS. 3A to 3K are illustrative only and are not tobe construed as limiting. For example, while themicro-electro-mechanical system illustrated with reference to FIG. 3A to3K is a combined pressure and acceleration sensor, in other embodimentsother kinds of micro-electro-mechanical devices may be formed, forexample pure acceleration sensors, pure pressure sensors, or any otherkinds of sensors, but not being limited to sensors. Examples for othermicro-electro-mechanical systems may for example include gas sensors. Inother embodiments, instead of a micro-electro-mechanical system otherstructures formed on a wafer, for example purely electrical devices, maybe used.

In FIG. 3A, a main wafer which has already undergone some processing isshown. The main wafer of FIG. 3 is an illustrative example for a mainwafer which may be provided to first material removal device 10 of FIG.1 as indicated by arrow 15 or provided to the method of FIG. 2 asindicated by arrow 20. The main wafer of FIG. 3 comprises a wafer body30, for example made of silicon. Various structures as depicted in FIG.3A are formed on a front side of the main wafer. To form thesestructures, conventional techniques like oxidation, lithography,etching, implantation, diffusion or epitaxy may be used. To give someexamples, the main wafer of FIG. 3A comprises doped regions 31, forexample boron-doped region, a region 33 where resistor structures areformed, an epitaxial layer 32, an oxide layer 316 on epitaxial layer 32,highly N-doped regions 36, metallizations or any other desiredstructures. The structures shown serve only as examples and may dependon the micro-electro-mechanical system to be manufactured. It should benoted that for ease of representation and for clarity's sake, in thefollowing FIGS. 3B to 3K not all reference numerals shown in FIG. 3A arerepeated, but only those reference numerals relevant to the explanationand illustration of a respective processing are shown.

In FIG. 3B, the result of a material removal as for example performed infirst material removal device 10 of FIG. 1 or at 21 of FIG. 2 is shown.In particular, a gap 34 in oxide layer 316 and in epitaxial layer 32 hasbeen produced, for example by etching. In the example of FIG. 3B, thematerial removal has been performed at a location corresponding to alocation between two doped regions 31 as shown. At this location, aswill be seen later, it is intended to produce an opening to generate forexample a tongue or a cantilever.

As illustrated in FIG. 3C, afterwards the gap 34 formed in FIG. 3B isfilled again with a filling material 35 in FIG. 3C. This filling inembodiments may serve as a passivation and may protect the front side ofthe wafer for example against chemical substances which are used forstructuring a backside of the wafer and which otherwise could entercavities in a support wafer, as will be explained later in more detail.

As filling material 35, in some embodiments a carbon-based material maybe used, for example amorphous hydrogenated carbon (a-C:H) ordiamond-like carbon (DLC). Alternative materials include photoresists orlacquers dissolvable via ultraviolet radiations or adhesives. Furtherexamples for filling materials include doped or undoped silicon oxide,silicon nitride, silicon oxynitride, aluminum oxide, high-k dielectricor low-k dielectric materials. With such material, in some embodimentsprior to filling the material in gap 34, a thin insolated oxide ornitride may be deposited which may serve as etch stop in laterprocessing. Portions of the filling material or materials remainingoutside gap 34 may for example be removed via lithography and etching.After this, as illustrated in FIG. 3D, a structured glass wafer 37 isbonded to the main wafer for example at N-doped portions 36. At thebonding locations, an oxide film which may be present on the main wafermay be removed. This bonding of structured glass substrate 37 is anexample of a processing which may be performed in carrier wafer bondingdevice 12 or at 23 of FIG. 2 and may, for example, be performed byanodic bonding. In other embodiments, instead of a glass wafer othermaterials may be used. Moreover, in other embodiments instead of anodicbonding other bonding techniques, in particular irreversible bondingtechniques, may be used. Glass wafer 37 in subsequent processing stepsmay serve as a permanent irreversible carrier supporting the main wafer.

Glass wafer 37, besides providing support for the main wafer, in someembodiments also protects the front side, for example against scratches,inducing the effect and contamination through processing devices,environment, chemical substances or the like.

This support provided by glass wafer 37 may then for example be used forthinning the wafer. As indicating by an arrow 38 in FIG. 3E, the waferbody 30 of the main wafer may be thinned, for example to a thicknessbelow 300 μm, below 100 μm or below 50 μm. Essentially, any thicknessmay be used with which, within a precision of the thinning process used,ensures that the structures formed at the front side of the main waferare not damaged by the thinning. For example, with some techniques, onlyabout 3 μm or about 5 μm wafer body thickness above the structuresformed, for example above doped regions 31, may remain. The thinning maybe performed using conventional grinding, polishing and/or etchingtechniques.

After this, in embodiments to prepare for a material removal from thebackside of the main wafer, a mask 39 for a subsequent etching isgenerated, as shown in FIG. 3F. The mask 39 may be deposited usingconventional techniques, for example chemical vapor deposition (CVD)techniques. In embodiments, though providing an irreversible bonding,like an anodic bonding, the bonding does not limit the possibletemperatures for the deposition of mask 39. Such temperatures may forexample be up to 550° C. in embodiments and are limited essentially bythe structures formed in the main wafer, for example a front sidemetallization. This is in contrast to conventional approaches forhandling thin wafers involving the use of a carrier and an adhesive formounting the wafer to the carrier. In such conventional approaches, thetemperature for subsequent treatments is limited by a temperaturestability of the adhesive, which may be considerably lower than 400° C.

Subsequently, an etching is performed. The result for the example ofFIG. 3 is shown in FIG. 3G. In particular, an etching sensitive to adoping concentration, also referred to as PN-etching, may be used whiche.g. leaves p-doped regions 31 unetched. For such an etchingconventional approaches may be used, for example an approach where thefront side of the main wafer is contacted. For such a contacting,contact holes (not shown in FIG. 3) may be provided in glass wafer 37.As the thickness of the wafer body 30 has been reduced previously,etching times may be reduced, compared to cases of thick wafers.Moreover, due to reduced etching depths underetching under mask 39 andspace needed for etch flanks may be reduced, which may reduce a requiredchip area which in turn may lead to an increase of the integrationdensity in some embodiments. The etching leads to spaces or cavities 310absent material.

Subsequently, the filling material 35 filled in the gap as illustratedand explained with reference to FIG. 3C is removed again, leading to thesituation as illustrated in FIG. 3H. Also, mask 39 is removed. Theremoval of the filling material may in particular may be performed by adry removal method like providing an oxygen plasma in case ofcarbon-based material. This in embodiments prevents liquids fromentering cavities of glass wafer 37.

This results in a tongue or cantilever 314 being “freed”, such thattongue 314 may for example be used as sensing element of an accelerationsensor.

The processes illustrated and explained with reference to FIGS. 3F to 3Hmay for example be performed using second material removal device 14 ofthe apparatus of FIG. 1 or may be performed at 25 of FIG. 2.

Subsequently, in some embodiments, an additional glass carrier wafer 311as illustrated in FIG. 3J may be bonded to the backside of the mainwafer, for example again using again anodic bonding. Afterwards, asillustrated to some extent in FIG. 3K, further processing steps likelaminating, bridge-cut sawing, cleaning, visual and quality control ordicing may be performed to form individual devices as shown in FIG. 3K.In the example of FIG. 3K, essentially an acceleration sensor usingtongue 314 as generally indicated by reference numeral 313 and apressure sensor using a membrane 315 as generally indicated by referencenumeral 312 may be formed. As already mentioned, this serves only as anexample, and in other embodiments other kinds of sensors or other kindsof micro-electro-mechanical systems may be formed. Generally, theprocessing illustrated with reference to FIG. 3J and 3K represents anexample for processing after second material removal device 14 of FIG. 1(as illustrated by an arrow 16) or after 25 of FIG. 2 (as illustrated byarrow 26).

In the example discussed above, in particular with respect to FIG. 3C, afilling material like a carbon-based material 35 was used to fill a gap34 formed at a front side of a wafer. This serves to ensure formation ofan opening (as shown in FIG. 3H), and by using a dry process for removalof the filling material in embodiments it is ensured that no liquidenters a cavity of glass wafer 37. In other embodiments, other fillingmaterials, for example doped or undoped silicon oxide, silicon nitride,silicon oxynitride, aluminum oxide, high-k dielectric or low-kdielectric materials may be used. In other embodiments, other approachesmay be used, some of which will now be discussed with reference to FIGS.4A and 4B.

In the alternative of FIG. 4A, instead of merely filling a carbon-basedmaterial into gap 34 of FIG. 3B, a thin oxide layer 40 is depositedprior to filling the gap with a carbon-based material 41 or any othersuitable material as discussed in FIG. 3C. In some embodiments, oxidelayer 40 may provide a better etch stop, for example for the etchingprocess illustrated with reference to FIG. 3G.

A further alternative is shown in FIG. 4B. Here, prior to depositingepilayer 32, an oxide material block 42 is structured on the main waferat the location of the gap 34. At oxide material block 42 then duringdeposition of epilayer 32 essentially no growth occurs. In this case, nomaterial removal as illustrated in FIG. 3B has to be performed. In someembodiments, a carbon-based layer 43 may then be deposited on top ofoxide 42 for passivation. For a material removal corresponding to whatwas described with reference to FIG. 3H, at first oxide 42 may beremoved for example by etching, e.g. wet etching, and then carbonpassivation 43 may be removed for example by an oxygen plasma.

Other techniques for filling gap 34 or for providing material which caneasily be removed later to provide openings through the substratewithout filling a cavity of glass wafer 37 with liquid may also be used.

Through the thinning of the main wafer as described above, in someembodiments, space within a package may be saved. In some embodiments,this may be used for stacking a further device on top of themicro-electro-mechanical system. A device according to an embodimentimplementing this is schematically shown in FIG. 5.

In FIG. 5, within a package 52, a micro-electro-mechanical system 50,for example manufactured as explained with respect to the embodiments ofFIGS. 1 to 4B, is provided. Micro-electro-mechanical system 50 may forexample comprise a pressure sensor and/or an acceleration sensor. On topof micro-electro-mechanical system 50, an application specificintegrated circuit (ASIC) 51, for example designed to evaluate signalsfrom micro-electro-mechanical system 50, is placed. In conventionalapproaches with thicker main wafers, conventionally it was often onlypossible to place ASIC 51 besides the micro-electro-mechanical system.ASIC 51 is coupled via bonds 53 with external contacts 54. With theapproach of FIG. 5, in some embodiments compact sensor devices may beprovided.

It is to be emphasized again that the embodiments discussed above serveonly as examples and are not to be construed as limiting. Rather, thedescribed embodiments serve to give a better understanding of somepossibilities for implementation of the techniques described herein.

What is claimed is:
 1. A method, comprising: irreversibly bonding a mainwafer to a carrier wafer at a front side of the main wafer, thinning themain wafer from a back side of the main wafer, and selectively removingmaterial from the back side of the main wafer.
 2. The method of claim 1,further comprising a forming a micro-electro-mechanical system in themain wafer.
 3. The method of claim 1, wherein the irreversibly bondingcomprises an anodic bonding.
 4. The method of claim 1, wherein the mainwafer comprises a semiconductor wafer, and the carrier wafer comprises astructured glass wafer.
 5. The method of claim 1, further comprising,prior to bonding the main wafer to the carrier wafer, selectivelyremoving material from the front side of the main wafer at one or morelocations to form one or more gaps where openings are to be formedthrough the main wafer.
 6. The method of claim 5, further comprisingfilling the one or more gaps formed by the removal of the material fromthe front side of the main wafer with a filling material.
 7. The methodof claim 6, wherein the filling material comprises a carbon-basedmaterial.
 8. The method of claim 6, wherein the filling materialcomprises an oxide.
 9. The method of claim 6, wherein selectivelyremoving material from the back side of the main wafer comprisesremoving the filling material.
 10. The method of claim 9, whereinremoving the filling material comprises performing a dry process. 11.The method of claim 1, further comprising, prior to the bonding of themain wafer to the carrier wafer, providing a material block on a frontside of the main wafer at a location where an opening is to be formedthrough the main wafer.
 12. An apparatus, comprising: a carrier waferbonding device configured to irreversibly bond a carrier wafer to a mainwafer at a front side of the main wafer, a thinning device configured tothin the main wafer from a back side thereof, and a material removaldevice configured to selectively remove material from the back side ofthe main wafer, the second material removal device being downstream ofthe thinning device.
 13. The apparatus of claim 12, further comprising afurther material removal device upstream of the carrier wafer bondingdevice, the further material removal device configured to removematerial at a front side of the wafer at a location where an openingthrough the main wafer is to be formed.
 14. The apparatus of claim 12,further comprising a filling device configured to fill gaps generated bythe material removal of the further material removal device with afilling material.
 15. The apparatus of claim 12, wherein the apparatusis configured to manufacture a micro-electro-mechanical system.
 16. Amicro-electro-mechanical system, comprising: a main wafer withmicro-electro-mechanical structures formed therein, a thickness of themain wafer being less than 300 μm, and a carrier wafer irreversiblybonded to a front side of the main wafer.
 17. The device of claim 16,wherein a thickness of the main wafer is less than 100 μm.
 18. Thedevice of claim 16, wherein the main wafer comprises at least one of anacceleration sensor or a pressure sensor.
 19. The device of claim 16,wherein the carrier wafer comprises a structured glass wafer.
 20. Thedevice of claim 16, further comprising: an application specificintegrated circuit mounted on top of the carrier wafer and the mainwafer, and a package packaging the application specific integratedcircuit, the main wafer and the carrier wafer.